The present invention relates to caustic solutions having reduced corrosive effect on nickel-based surfaces. In particular, this reduction in corrosion is due to the addition of a small amount of hydrogen or a metal hydride to the caustic solution. The hydrogen or metal hydride is added during the process for manufacturing caustic soda, and preferably during the concentration of the caustic solution by evaporation of water from the solution.
Caustic soda (NaOH) is commercially manufactured by the electrolysis of brine in chlor-alkali diaphragm or membrane cells followed by the concentration of the catholyte cell liquor to evaporate water and to precipitate sodium chloride from the solution. The evaporation process typically employs a multiple stage evaporator fabricated from nickel, nickel alloys or materials such as E-Brite. Such evaporators operate at elevated temperatures of from about 100.degree. C. to about 200.degree. C. A considerable amount of fluid turbulence is also present in the evaporator system. Under these conditions, the observed corrosion rate of nickel or nickel alloy evaporator surfaces can exceed 0.2 g/m.sup.2 hr. In addition, ancillary nickel or nickel alloy piping and components which are exposed to such caustic solutions also undergo a substantial amount of corrosion and/or erosion.
Corrosion in caustic manufacturing systems represents a significant commercial problem since it can reduce the effective life of system components and, in addition, results in a sodium hydroxide product having significant amounts of dissolved nickel, i.e. 2 ppm or more. This is undesirable since bleaching chemicals prepared from caustic soda containing high concentrations of dissolved nickel are discolored in appearance and are chemically unstable.
Prior art attempts to solve the corrosion problem have primarily focused on the necessity for eliminating or reducing the amount of sodium chlorate in the system. Sodium chlorate is present in diaphragm cell liquor as an impurity in concentrations generally less than about 1%. Sodium chlorate is a strong oxidizing agent, and even amounts as little as 0.02% can cause significant corrosion problems for nickel-based materials. Sodium sulfite has been widely used for sodium chlorate removal. The use of sodium sulfite for this purpose, however, is associated with several disadvantages such as its cost, the necessity for significant capital additions to the plant, and product contamination.
The use of hydrogen to reduce the chlorate content of chlorate-containing solutions is disclosed in Netherlands patent application No. 7603314, published Oct. 4, 1977, wherein hydrogen is reacted with the solution under high temperature and pressure conditions in the presence of a carbon-supported ruthenium catalyst to produce sodium chloride and water as illustrated by the following reaction: ##STR1## This process is also disadvantageous since it requires approximately stoichiometric amounts of hydrogen, and utilizes elevated temperature and pressure conditions and expensive catalytic materials.
U.S. Pat. No. 4,282,178 discloses the use of hydrazine in relatively small quantities, i.e. less than 40 ppm, to inhibit corrosion of nickel-containing metal surfaces during caustic evaporation at elevated temperatures. Although the hydrazine is indeed effective at reducing the corrosion rate of nickel-containing surfaces and in improving the purity of the concentrated caustic soda product, the use of hydrazine in such systems is associated with several disadvantages. For instance, hydrazine is extremely explosive and must be handled with great care. This requirement necessitates the installation of expensive equipment which must be used to handle hydrazine, and requires the use of numerous safety precautions in the plant. The instability of hydrazine places severe limitations on its use in the cell room prior to evaporation and further limits its safe use in the final product.
It is therefore a principal object of the present invention to provide an efficient, safe and relatively inexpensive method for suppressing corrosion of nickel-based materials used in caustic manufacturing systems.